My boyfriend and I want to get married but he is Sikh and I am Catholic. He is willing to marry in the Catholic Church, but can we also get married through his church or somehow do it at the same time with both priests?
You will need to obtain permission from your local bishop to marry a non-Catholic. While it is possible for a couple in a mixed marriage to be married in a non-Catholic ritual if the Catholic has been given a dispensation for such a marriage by the local bishop, you cannot have two weddings and you cannot have two presiders at the marriage both obtaining your consent to marry. At most, with the proper ecclesial permission from the diocese, either the Sikh clergyman can say a prayer for you at the Catholic wedding or the Catholic priest can say a prayer for you at a Sikh wedding (assuming you can find a Catholic priest who will agree to participate).
This answers the question of how to plan such a wedding. Whether a Catholic should marry a non-Christian is an entirely different question. For more information, please see the links below.